Medusa

Medusa is a Greek mythological figure described as a winged human female with living venomous snakes in place of hair. Those who gazed upon her face would turn to stone.

With her older sisters Stheno and Euryale, they made up the three Gorgon Sisters who inhabited the Shapeless Isle. She came into existence thanks to the thoughts and prayers of humans, who desired a perfect goddess, but she was cursed for her beauty and turned into a monster hated by all. She is thus not a Heroic Spirit but a monster closer to a Divine Spirit.

Due to her nature, she can manifest through different aspects depending on which disposition is predominant (more or less monstrous)

She is the Rider-class Servant of Shinji Matou in the Fifth Holy Grail War of Fate/stay night.

She only truly acts as Sakura's Servant when their connection is revealed in Heaven's Feel.

She is a Lancer-class Servant. Medusa is Blackened, a process by which a being is tainted by an outside force, causing their personality to be reversed toward the orientation of evil.

In this singularity Medusa manifests as two separate beings:

One is Ana, a Lancer-class Servant without a Master. This incarnation represents the young Medusa, from a time when she wasn't known as the monster inhabiting the Shapless Ile. Though still wary of humans for casting her away, she is much closer to the perfect Goddess she was supposed to be.

The other one is Gorgon, an Avenger-class Servant. This incarnation represents Medusa as a man-eating monster. The burning hatred against the humans that exiled her and her sisters led her to become the very monster they accused her to be. Gorgon wields Tiamat's Authority which grants her the ability to generate Demonic Beasts.